1. Field of the Invention
The present invention relates to seals for rotating shafts to restrict fluid flow between rotating and stationary members; more particularly, it is related to a face seal which has improved sealing characteristics and which deliberately induces turbulent flow along the seal gap.
2. Description of the Known Art
The employment of seals along the rotating shaft for large machines such as gas turbines or steam turbines provides complex sealing problems. Such machines have traditionally utilized labyrinth type seals at critical sealing locations such as at the compressor discharge of a gas turbine or at the ends of a steam turbine shaft. Labyrinth type seals normally result in relatively high leakage, inasmuch as a labyrinth seal is usually designed such that for axial flow the radial clearance is 1 mil (0.001 inches) of clearance per inch of shaft diameter. Thus, for a 20 inch diameter shaft, 20 mils of clearance would normally be provided in a labyrinth seal. In gas turbines, 20 inch diameter shafts are not uncommon, whereas in certain steam turbines, 60 to 80 inch diameter shafts require sealing and a 60 to 80 mil clearance may well be provided. On the other hand, seals such as hydrostatic face seal assemblies are normally intentionally designed so that laminar flow occurs in the seal gap. Laminar flow is achieved by providing a converging flow path on the seal from a high pressure region in the machine toward a lower pressure region, with a face clearance on the order of 0.0005 inches (0.5 mil inches) and smaller. Seals with such minimal face clearances are difficult if not impossible to use in such applications, because such seals are essentially intolerant to distortions in geometry which can occur over a seal lifetime and during transient conditions where, for example, pressure and temperature differences may vary significantly. As a result, hydrostatic face seals have not been employed successfully for such applications. Seal designs such as the seal described in my U.S. Pat. No. 5,284,347, issued on Feb. 8, 1994, which had been proposed for such applications in such seal designs, include rather complex gas bearing faces in flow paths as well as complex centering means to attempt to minimize seal distortion. While not expressly discussed, such seals are not designed to deliberately provide turbulent flow in the seal gap.